http://dev.w3.org/csswg/css3-background/#the-border-radius says:
# Backgrounds, but not the border-image, are clipped to the
# appropriate curve (as determined by ‘background-clip’). Other
# effects that clip to the border or padding edge (such as
# ‘overflow’) also must clip to the curve.
What does it mean for 'overflow: scroll' or 'overflow: auto' to clip
to the curve? I have no idea how that's supposed to work; there
aren't standard mechanisms for drawing curved scrollbars.
Then it goes on to say:
# It is recommended that the UA style sheet apply overflow: hidden
# to elements (such as the <img> element in HTML) that are
# expected to be replaced elements so that their corners
# automatically trim to the border radius.
I'm pretty skeptical of this advice; 'overflow: hidden' is a pretty
heavyweight mechanism (it involves the ability to scroll
programatically). I suspect we'd be unable to make this change
because it would cause a performance regression, although I haven't
tried.
-David
--
L. David Baron http://dbaron.org/
Mozilla Corporation http://www.mozilla.com/